1) Field of the Invention
This invention relates to a novel color-developing composition comprising a multivalent-metal-modified salicylic acid resin and also to an aqueous suspension of the composition. The color-developing composition is usable in pressure-sensitive copying paper sheets, heat-sensitive recording paper sheets, copying ink compositions, color-developing agents for transfer-type copying paper sheets, and the like.
2) Description of the Related Art
Pressure-sensitive copying paper sheets are also called "carbonless copying paper sheets". They produce a color by mechanical or impactive pressure, for example, by writing strokes or typewriter impression, thereby enabling one to make a plurality of copies at the same time. Among such pressure-sensitive copying paper sheets, are those called "transfer type copying paper sheets", those called "self-contained copying paper sheets", etc. Their color-producing mechanisms are each based on a color-producing reaction between an electron-donating colorless dyestuff precursor and an electron-attracting color-developing agent.
In general, a pressure-sensitive copying paper sheet is formed of a sheet (CB-sheet), which is coated with microcapsules of a non-volatile organic solvent containing an electron-donating organic compound (pressure-sensitive dyestuff), and another sheet (CF-sheet), which is coated with an aqueous coating formulation containing an electron-attracting color-developing agent, with their coated sides maintained in a face-to-face contiguous relationship. The microcapsules are ruptured by the above-described printing pressure, so that the pressure-sensitive dyestuff solution is caused to flow out into contact with the color-developing agent to develop a color. By changing the combination of a microcapsule layer, which contains a pressure-sensitive dyestuff, and a color-developing layer, it is possible to make a plurality of copies or to produce pressure-sensitive copying paper sheets capable of producing a color individually (SC-sheets).
Taking a pressure-sensitive copying paper of the transfer type by way of example, it will be described with reference to FIG. 1 which is a schematic cross-sectional view showing the structure of the illustrative pressure sensitive copying paper sheet.
The back sides of a CB-sheet 1 and CF/CB-sheet 2 are coated with microcapsules 4 which have diameters of from several micrometers to somewhat greater than 10 micrometers obtained by dissolving a colorless pressure-sensitive dyestuff precursor in a non-volatile oil and then encapsulating the resultant pressure-sensitive dyestuff precursor solution with a high molecular film such as gelatin film. On the other hand, the front sides of the CF/CB-sheets 2 and a CF-sheet 3 are coated with a coating formulation containing a color-developing agent 5 which has such properties that upon contact with the pressure-sensitive dyestuff precursor, the color-developing agent 5 undergoes a reaction with the dyestuff precursor, thereby causing the dyestuff precursor to product its color. In order to make copies, they are stacked in the order of the CB-sheet, (CF/CB-sheet) and CF-sheet with the sides coated with the dyestuff precursor maintained in contiguous relation with the sides coated with the color-developing agent. When a pressure is applied locally by a ball-point pen 6 or a typewriter, the capsules 4 thereunder are ruptured. As a result, the solution containing the pressure-sensitive dyestuff precursor is transferred to the color-developing agent 5 so that one or more copied records are obtained.
Illustrative colorless or light-colored dyestuff precursors usable in such pressure-sensitive copying paper sheets include:
Triarylmethanephthalide compounds such as Crystal Violet lactone. PA0 Fluoran compounds such as 3-dibutylamino-6-methyl-7-anilinofluoran. PA0 Pyridylphthalide compounds. PA0 Phenothiazine compounds. PA0 Leucoauramine compounds.
One or more dyestuff precursors selected from these dyestuff precursors are dissolved in a hydrophobic high-boiling-point solvent and microencapsulated for us in the production of pressure-sensitive copying paper sheets.
As electron-attracting color-developing agents, there have been proposed (1) inorganic solid acids such as acid clay and attapulgite, as disclosed in U.S. Pat. No. 2,712,507; (2) substituted phenols and diphenols, as disclosed in Japanese Patent Publication No. 9309/1965; (3) p-substituted phenol-formaldehyde polymers and multivalent-metal-modified products thereof, as disclosed in Japanese patent Publication No. 20144/1967; (4) metal salts of aromatic carboxylic acids, as disclosed in Japanese Patent Publication Nos. 10856/1974, 25174/1976 and 1327/1977, and Japanese Patent Laid-Open Nos. 148614/1979, 84045/1987, 132857/1988, 112537/1988 and 91042/1990. Some of them have already been employed commercially.
Performance requirements which a color-developing sheet should satisfy include (1) high density color marks produced at room temperature, (2) small density reduction in the produced color marks during long-term storage, (3) high speed color-developing of the color marks especially at low temperatures, (4) reduced yellowing of paper surface during storage or upon exposure to radiant rays such as sunlight, (5) high resistance of the produced color marks to disappearance or fading upon contact with water or a plasticizer, (6) high resistance of the produced color marks to fading upon exposure to radiant rays such as sunlight.
Color-developing agents which have been proposed to date and sheets coated with such conventional color-developing agents have both advantages and disadvantages as will be described next.
1. Inorganic solid acids:
For example, inorganic solid acids are inexpensive but adsorb gas and moisture from the air during storage. They hence result in yellowing of paper surfaces and reduced color-producing performance. Color marks produced using inorganic solid acids undergo substantial fading when exposed to radiant rays such as sunlight.
2. Substituted phenols:
Substituted phenols have insufficient color-producing produce ability and produced color marks which have a low color density. At low temperatures, the color-developing speed is low. 3. p-Substituted phenol-formaldehyde polymers:
p-Phenylphenol-novolak resins which are primarily employed as p-substituted phenol-formaldehyde polymers are excellent in the density of color marks produced therefrom, the color-developing speed at low temperatures and the resistance to water or a plasticizer, but paper sheets coated with them undergo yellowing and color marks produced therefrom fade significantly upon exposure to radiant rays such as sunlight or during storage (especially, by nitrogen oxides in the air).
4. Metal salts of aromatic carboxylic acids:
As color-developing agents capable of improving the drawbacks of conventional color-developing agents, some metal salts of aromatic carboxylic acids, especially metal salts of salicylic acid derivatives have been proposed. When these color-developing agents are used in copying or recording paper sheets, the coated paper surfaces are imparted with improved yellowing resistance but the low-temperature color-developing ability, water or plasticizer resistance, light fastness and the like, which have heretofore been considered to present problems, cannot be considered improved.
Some methods have been proposed with a view toward improving these drawbacks. For example, with a view toward improving light fastness or water or plasticizer resistance, Japanese Patent Publication No. 1195/1980 (which corresponds to U.S. Pat. No. 4,046,941) proposes to use a compatible resin in combination with a salicylic acid compound. Such a method is certainly effective for the improvement of waterproofness and light fastness but is still insufficient with respect to the color-developing speed at low temperatures and the density of color marks produced at low temperatures.
Effects of a salicylic acid compound as a color-developing agent are dependent on its substituent group or groups. Therefore, color-developing ability is generally low even when a metal salt of salicylic acid is used in combination with a compatible resin. Introduction of at least one aromatic substituent group into the skeleton of salicylic acid is therefore an essential requirement for salicylic compounds to be used in accordance with such a method.
In attempts to improve the low-temperature color-developing ability and the water or plasticizer resistance, some methods have been proposed in recent years to resinify salicylic acid and to use its metal-modified products.
Examples of such attempts include metal-modified polybenzylsalicylic acids obtained from salicylic acid and a benzyl halide, as disclosed in Japanese Patent Laid-Open No. 132857/1988 (U.S. Pat. No. 4,879,368); metal-modified salicylic acid resins obtained from salicylic acid and styrenes, as disclosed in Japanese Patent Laid-Open No. 112537/1988 (U.S. Pat. No. 4,929,710); and metal-modified salicylic resins formed from salicylic acids and various benzyl derivatives, as proposed by the present inventors and disclosed in (1) Japanese Patent Laid-Open No. 186729/1988, (2) Japanese Patent Laid-Open No. 254124/1988, (3) Japanese Patent Laid-Open No. 289017/1988, and (4) Japanese Patent Laid-Open No. 56724/1989 and (5) Japanese Patent Laid-Open No. 77575/1989, which in combination correspond to U.S. Pat. No. 5,023,366.
It is stated as an advantage that the low-temperature color-developing speed and waterproofness are generally improved to significant extents when these metal-modified salicylic acid resins are used as color-developing agents.
There is, however, an outstanding demand for further improvements in light fastness with respect to the above-described multivalent-metal-modified salicylic acid resins. It is known, as a matter of fact, that the light fastness of color marks produced by using such a color-developing agent varies fractionally depending on the structure, molecular weight distribution and the like of the resin. Namely, the light fastness of produced color marks tends to improve when there is a substituent group such as an alkyl group at the .alpha. carbon of a benzyl compound relative to salicylic acid in the structure of the resin. Further, random bonding is generally considered more preferable than linear bonding in the manner of bonding of a resin, and broader molecular weight distribution is generally considered more preferable.
Based on those findings, the present inventors previously proposed a process for the production of an improved multivalent-metal-modified salicylic acid resin in Japanese Patent Laid-Open No. 133780/1989 (U.S. Pat. No. 4,952,648). According to the process, a styrene is reacted with a salicylic acid ester to obtain a salicylic acid ester resin having a broad molecular weight distribution. After the salicylic acid ester resin is hydrolyzed, the resulting salicylic acid resin is reacted with a multivalent metal salt so that a multivalent-metal-modified salicylic acid resin is obtained. In the resin obtained in accordance with this process, its structure and molecular weight distribution have been improved in a preferred direction. There is, however, an outstanding demand for still further improvements.
To produce a pressure-sensitive copying paper sheet using a color-developing agent, the color-developing agent is generally wet-ground in the presence of a surfactant so that the color-developing agent is formed as fine particles having a particle size of 1-10 .mu.m into an aqueous suspension. Upon formation of the suspension, a dispersant is also used. The selection of a combination of particles to be dispersed and a dispersant for the provision of a good dispersion system practically one relies upon experience in many instances, since there is no general rule to follow. When a dispersant is chosen, it is necessary to take into account not only its dispersing ability but also its interaction with the dispersed particles. For example, for phenol-formaldehyde condensation products which have been employed as color-developing agents in pressure-sensitive copying paper sheets, an anionic high molecular surfactant of the polycarboxylic acid type, specifically the sodium salt of a maleic anhydride-diisobutylene copolymer is usually used as a dispersant. If this dispersant is used upon formation of the color-developing composition, which comprises the above-described multivalent-metal-modified salicylic acid resin, into an aqueous suspension, a complex is, however, inconveniently formed between the multivalent metal and the carboxylic acid salt, resulting in a substantial reduction in the dispersing ability and dispersion stability, the production of stable foams, changes in the physical properties of the color-developing agent due to modifications of the multivalent-metal-modified salicylic acid resin as a dispersoid, etc. It is therefore impossible to obtain any practically usable aqueous suspension. Salts of naphthalenesulfonic acid-formaldehyde condensation products, salts of ligninsulfonic acid, and the like--which were previously employed for color-developing agents of the phenol-formaldehyde condensation products--include those capable of showing dispersing ability for color-developing compositions comprising a multivalent-metal-modified salicylic acid resin. When they are employed in pressure-sensitive copying paper sheets, the pressure-sensitive copying paper sheets are accompanied by a drawback such as coloration, light yellowing or the like of the paper surfaces due to the dispersants themselves so that such dispersants substantially lack practical utility.
It is accordingly not easy to combine a color-developing composition, which comprises the above-described multivalent-metal-modified salicylic acid resin, with a suitable dispersant into an aqueous suspension having good quality in various properties such as dispersibility, stability and color-developing ability.